Several systems have already been proposed in order to regulate the luminosity of a discharge lamp as for example a fluorescent tube by acting for example on a manual control which in turn controls the conduction of a thyristor during a predetermined time lapse. If the luminous intensity of the discharge lamp must be automatically controlled, for example in order to form an animated image by a multiplicity of lamps from the input of a video signal, one might call on the technique which consists of energizing each lamp by a high frequency generator such as is described in the published European patent application EPO No. 109 671 (U.S. Pat. No. 4,559,480). In this technique the current flowing through the lamp is constituted by the juxtaposition of reference periods, each comprising a plurality of cycles. The intensity of the light emitted is varied by means of an element located in series in the energizing system of the lamp which enables controlling its "on" time by the inhibition of a variable number of cycles contained in each reference period.
The system the operation of which has just been outlined has the advantage of presenting an almost instantaneous turn-on of the lamp and good light efficiency. It however has the disadvantage of necessitating for each lamp a current stabilizing element (ballast) at the same time as requiring a chopped high voltage on the order of 400 volts permanently applied to the terminals of the lamp provided with its ballast during periods when it is to be excited. Such a system presents the difficulty of regulating the discharge current in the lamp.
The same system presents furthermore the disadvantage of requiring the employment of fluorescent tubes each provided with two pre-heating filaments. As a result thereof, there is required the use of an isolating transformer for each of the tubes, thereby leading to complications and greater difficulties for the realization of the overall system.
Various documents which came to light during preliminary research will now be given a short analysis.
U.S. Pat. No. 3,590,316 describes an apparatus serving as ballast for a plurality of discharge lamps. It concerns however a well-known type of turn-on system where the firing and maintenance of the arc are effected by means of a single inductance coil. Contrary to this and as will appear hereinafter, the energizing system according to this invention requires two distinct energizing sources, one serving to fire the arc and the other for its maintenance, this for the purpose of controlling the luminosity of the lamp over a wide range. The cited patent does not separate the creation of the arc and thereafter maintenance of such arc by two different sources, does not permit extended regulation and furthermore does not permit the utilization of the system to energize a tube forming part of a point of a moving image.
U.S. Pat. No. 4,132,925 describes a system comprising a starting circuit and a DC ballast in order to energize a discharge lamp. In this system, as soon as the discharge has begun, the starting circuit becomes inactive and it is the amplitude of the DC current which controls the luminosity of the lamp. One is thus concerned with the regulation of the luminous intensity by variation of the current amplitude and not by variation of the duration of a constant current as is the case in the present invention. In this latter, one is concerned principally with the use of the tube as a matrix component of a video image and for that it is necessary to refresh the luminous points comprising such image at predetermined periods, this being non-realizable in the arrangement of the cited patent.
U.S. Pat. No. 4,219,760 describes a manual system of regulation of the luminous intensity of a discharge lamp. It will be noted however that the energization, moreover summarily described, does not consist of a continuous energization, but a pulsed energization, which is specifically avoided in the present invention.
Finally, French patent 2,397,768 (corresponding to U.S. Pat. No. 4,158,793) likewise fails to show periodic firing or maintenance pulses as has been mentioned hereinabove. Furthermore, one will not find a continuous current source but rather a voltage source. Finally, no method is shown for independently controlling the luminous intensity of each of the three tubes represented which are all controlled at the same time by means of a single control source.